The proposed project aims to develop a compact and light-weight robotic prosthesis, with the objective of significantly enhancing the health and life quality of the 400,000 transtibial (i.e., below-knee, BK) amputees in the United States. In human locomotion, the ankle plays an important energetic role, and supplies substantially more positive power than the knee and hip. However, in the majority of existing BK prostheses, the prosthetic ankle joints are energetic passive, only storing and dissipating energy in use. The inability to generate power significantly impairs the prosthesis' capability of restoring the locomotive functions for the amputee user. Clinical studies indicate:that amputees wearing passive BK prostheses exhibit asymmetric gait kinematics and expend significantly more energy in comparison with healthy subjects. To address this issue, the investigators will explore an innovative chemo-fluidic sleeve muscle actuation system to power the BK prostheses. Two key elements form the basis of this new system: (1) Sleeve muscle actuator, an innovative muscle actuator with very high power density (at least ten times higher than DC motor); (2) Highenergy- density pneumatic supply, which is able to store a large amount of energy within a highly compact package. With this high-performance actuation system, the prosthesis is anticipated to meet the demanding power requirements imposed by the human locomotion, while minimizing the prosthesis weight and height to enlarge the amputee population that can benefit from the new powered BK prosthesis. The breakthrough in actuation technology will be complemented with a novel intent recognition and motion control algorithm, which is anticipated to provide the user's desired motion in a highly interactive manner. This control system identifies the desired locomotive pattern based on the user's inferred motion intent, and generates the desired joint torque command though ah impedance-based motion controller that enables physical interaction between the user and the prosthesis. The robotic prosthesis and the corresponding control system will be further tested to characterize its biomechanical benefits relative to state-of-the-art passive BK prostheses.